1. Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus utilizing the optical scanner. More specifically, the present invention relates to an apparatus such as a laser beam printer or a digital copying machine, in which a multi-emission semiconductor laser having plural light emission portions is used as a light source for achieving high speed and high density printing.
2. Related Background Art
FIG. 7 shows a cross section of a main portion of a conventional multibeam scanning optical system in the main scanning direction, in which plural light sources are used and each of the light sources has two light emission portions.
In FIG. 7, a multi-emission semiconductor laser 91AB having two light emission points emits two light beams, which are collimated substantially by a collimator lens 92. Then, they are focused in the sub scanning direction by a cylindrical lens 94 that has refraction power only in the sub scanning direction, shaped by an aperture 93, and are made an image like a line-extending in the main scanning direction in a vicinity of a deflection surface 95a of a polygon mirror 95 that is a light deflection device.
These two light beams are reflected and deflected by the polygon mirror 95 rotating in the direction of an arrow 95b at a constant speed. Then, they are focused by a scanning lens 96, which has the fxcex8 property and includes lenses 96a and 96b, so as to be spots on a photosensitive drum surface 97 that is a surface to be scanned, for scanning the surface at a constant speed in the direction of an arrow 97b. 
Furthermore in FIG. 7, another multi-emission semiconductor laser 91CD emits light beams, whose paths are bent by a light path combining prism 98 toward the deflection surface. Then, the light beams are focused onto the surface to be scanned by a polygon mirror 95 and scans the same similarly.
Here, optical axes of the multi-emission semiconductor lasers 91AB and 91CD are positioned so as to form a predetermined angle between them in the main scanning direction and are set so that the light beams meet substantially at the incident aperture 93.
In a multibeam optical scanner, if the plural light emission portions A and B are aligned vertically in the sub scanning direction as shown in FIG. 8, the distance between plural scan lines in the sub scanning direction on the photosensitive drum surface becomes much larger than a printing density. Therefore, the plural light emission points A and B are usually placed in a slanting direction to the sub scanning direction as shown in FIG. 9, and the slanting angle xcex8 is adjusted so that the distance between the plural scan lines in the sub scanning direction on the photosensitive drum surface is adjusted precisely to the printing density.
In addition, since scan lines corresponding to the light beams emitted by the light emission portions C and D are added, these four spots from the four light emission points are arranged on a line alternately on the photosensitive drum surface 97 as shown in FIG. 12.
The alternate arrangement can reduce the distance between the light beams in the main scanning direction on the polygon mirror deflection surface 95a, so that displacement between image forming positions of light beams in the main scanning direction from plural light sources can be suppressed to small amount.
It is because that a beam occupying width in the main scanning direction in the case without the alternate arrangement becomes three times that in the case of two beams, while the beam occupying width in the case with the alternate arrangement is suppressed to one and a half of that in the case of two beams.
Furthermore, the reason why the above-mentioned image misregistration in the main scanning direction occurs is explained in detail in JP 2001-228422 A (columns 8-12).
When placing the optical scanner inside the image forming apparatus, laser oscillation may become unstable if the light beam that entered the photosensitive drum surface is regularly reflected by the photosensitive drum surface and goes back to the light source, i.e., the multi-emission semiconductor laser. It is also possible that the regularly reflected light may reach the photosensitive drum surface again after being reflected by a surface of an optical component inside the scanner, resulting in generation of a ghost. Therefore, up to now, it is taken into consideration that the light beam entering the photosensitive drum surface is slanted by an angle xcex1 from the normal of the photosensitive drum surface so that the light reflected by the photosensitive drum surface can not go back to the optical system or the light source again as shown in FIG. 10.
However, there is a problem that if the above-mentioned structure is adopted in which the incident light beam is slanted to the photosensitive drum, image forming positions of plural scan lines in the main scanning direction on the photosensitive drum surface become different from each other as shown in FIG. 11.
In order to solve this problem, there is a method disclosed in JP 2001-59945 A. In this method, plural light beams entering the scanning lens are made convergent light beams or divergent light beams so that image misregistration between plural generated light beams is canceled.
According to the method proposed above, the misregistration between the plural light beams in the main scanning direction can be corrected almost perfectly.
However, since actual products have errors of component dimensions in manufacturing process and positioning errors in assembly, it takes long time in adjustment processes for precise positioning.
Especially, if the position of image formation by the light beam emitted from the optical scanner is not identical to a position on the photosensitive drum surface, i.e., if there is a positioning error in the optical axis direction, the above-mentioned effect is not obtained so that increase of the spot diameter and misregistration of image forming position of plural beams in the main scanning direction occur, resulting in deterioration of image quality.
The present invention has been made in view of the above, and an object thereof is to provide an optical scanner having high performance, especially a multibeam optical scanner utilizing plural light sources having plural light emission points by reducing influences of image misregistration between scan lines in the main scanning direction even in the case where there is positioning error between the optical scanner and a drum.
It is another object of the present invention to realize cost reduction by reducing work items of positioning adjustment and by reducing the work time.
In order to achieve the above-mentioned objects, according to one aspect of the present invention, there is provided an optical scanner including:
a plurality of light sources each of which has a plurality of light emission points;
a first optical system for leading a plurality of light beams emitted by the plural light sources to a deflection surface of deflection means; and
a second optical system for focusing the plural light beams deflected by the deflection means on a surface to be scanned so as to form a series of spots with a predetermined distance in the sub scanning direction, in which
focusing positions of the spots on the surface to be scanned taken along the sub scanning direction are arranged sequentially toward either one sense of the main scanning direction, and a distance occupied by all of the spots in the main scanning direction is shorter than a distance in the main scanning direction between lines that are set so as to pass the spots from a single light source among the plural light sources and extend over the distance occupied by all of the spots in the sub scanning direction.
In further aspect of the optical scanner, it is preferable that the focusing positions of the spots on the surface to be scanned taken along the sub scanning direction are arranged sequentially toward either one sense of the main scanning direction, a spot from one light source is disposed between spots from another light source in the sub scanning direction, and a distance L in the main scanning direction between two spots disposed at an innermost side and emitted by different light sources among the plural light sources satisfies the following inequality:
Wxe2x89xa7L greater than W/nxe2x80x83xe2x80x83(1) 
where W represents a distance in the main scanning direction between spots from a single light source among the plural light sources, and n represents the number of light sources having plural light emission points.
In further aspect of the optical scanner, it is preferable that the focusing positions of the spots on the surface to be scanned taken along the sub scanning direction are arranged sequentially toward either one sense of the main scanning direction, a spot from one light source among the plural light sources is not disposed between spots from another light source in the sub scanning direction, and a distance L in the main scanning direction between neighboring spots emitted by different light sources among the plural light sources satisfies the following inequality:
W greater than Lxe2x89xa70xe2x80x83xe2x80x83(2) 
where W represents a distance in the main scanning direction between spots from a single light source among the plural light sources.
In further aspect of the optical scanner, it is preferable that the optical scanner further includes light path conversion means for deflecting light beams emitted by the plural light sources to a predetermined outgoing direction.
In further aspect of the optical scanner, it is preferable that at least one of the plural light sources is a semiconductor laser having a plurality of light emission points.
In further aspect of the optical scanner, it is preferable that the plural light sources and the first optical system are arranged so as to form an opening angle in the main scanning direction so that the distance in the main scanning direction between the spots of the plural light sources becomes a predetermined distance.
In further aspect of the optical scanner, it is preferable that at least one of the plural light sources is disposed at a position that does not agree to the optical axis of the second optical system in the main scanning direction so that the distance in the main scanning direction between the spots of the plural light sources becomes a predetermined distance.
In further aspect of the optical scanner, it is preferable that the light path conversion means is a combining prism made of plural prisms bonded together.
In further aspect of the optical scanner, it is preferable that the apical angle of the prism constituting the light path conversion means is set so that the distance between the spots of the plural light sources in the main scanning direction on the surface to be scanned becomes a desired distance.
In further aspect of the optical scanner, it is preferable that the light path conversion means is a plane member having reflection function.
In further aspect of the optical scanner, it is preferable that an aperture is disposed in the light path in the vicinity of the deflection surface of the deflection means.
According to another aspect of the present invention, there is provided an optical scanner including:
a first optical system for leading at least three light beams to a deflection surface of deflection means;
a second optical system for focusing sequentially as spots the at least three light beams deflected by the deflection means on a surface to be scanned with a predetermined distance in the sub scanning direction, wherein
focusing positions of the spots on the surface to be scanned taken along the sub scanning direction are arranged sequentially toward either one sense of the main scanning direction, and a distance occupied by all of the spots in the main scanning direction is shorter than a distance of the spots at both ends of a virtual line on which all the spot are aligned.
In further aspect of the optical scanner, it is preferable that it further comprises a monolithic light source having the light emission points for emitting the at least three light beams on the same substrate.
In further aspect of the optical scanner, it is preferable that it further comprises a hybrid light source the light emission points of at least three light beams on different substrates.
In further aspect of the optical scanner, it is preferable that the first optical system has a function for converting the plural light beams into convergent light beams.
According to still another aspect of the present invention, there is provided an image forming apparatus including:
the above-mentioned optical scanner;
a photosensitive member arranged on the surface to be scanned;
a developing unit for developing an electrostatic latent image formed on the photosensitive member by the scanning light beams provided by the optical scanner as a toner image;
a transferring unit for transferring the developed toner image onto a transferring material; and
a fixing unit for fixing the transferred toner image on the transferring material.
According to yet still another aspect of the present invention, there is provided an image forming apparatus including:
the above-mentioned optical scanner; and
a printer controller for converting code data entered from an external device into an image signal and for entering the image signal into the optical scanner.